| Author: IMS Company |
| Maximizing
the Value of Regrind |
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| With resin prices continuing to go up, the use of
regrind is becoming a more viable tool for the molder. The nature of plastic
processing ensures that we will probably always have regrind, whether it comes
from start up, trimmed flash, rejected parts or purging. Following are some
guidelines on the proper use of regrind. |
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Heat History
Heat history of the regrind being used is important. If the material was
processed outside its temperature range, beware: stabilizers, lubricants,
fillers, and even color can be chemically altered. This will alter the physical
properties of the part being molded. For example, you may experience uneven
part shrinkage, poor color dispersion, voids and warpage. |
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Regrind Size
How the regrind is introduced back into the mixing process also should be
carefully examined. The mixture should be of the same bulk density for each
cycle. |
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What is the size of the regrind pellets? If too large, the smaller virgin
pellets will drop through the feed throat faster than the regrind does. If too
small, there may be fines (dust-like particles) that can lead to bridging in
the feed hopper. (If bridging is a constant problem, I invite you to check out
IMS’ innovative Spiral curve design hopper. Invented by IMS, this hopper makes
the flow of material more efficient, eliminating bridging.)
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A 1/4” screen in your grinder does not guarantee that every granule is exactly
1/4”. Whenever parts, sprues and runners go through a grinder there is the
potential to get a wide range of granule sizes. Sizes could be anywhere from
1/4” to fines.
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Fines are produced any time you produce regrind. They can be found not only all
over the grinder but also on the molding machine gates, in the extruder area,
at the feed hopper, on the floor and in the regrind. Fines can be a major
contamination problem because, during the plasticizing cycle, the screw will
melt these particles much earlier than the virgin resin. This can cause
streaks, splay, and black and white specks in the finished product.
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Keep in mind that extruding screws are generally designed to convey pellets,
not irregular chips, flake, fines or strings. If you want near-perfect
proportions, have the regrind re-pelletized. Not only will re-pelletizing give
you uniform size, but it will also allow for the regrind to be melt-filtered –
removing any non-plastic contamination.
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To maximize the consistency of the size of the granules, the grinders should be
cleaned properly, and well maintained with blades sharpened and adjusted on a
regular basis. Blade sharpening should be professionally done. (IMS has been
offering blade sharpening services for years.)
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There is a wide range of grinders/granulators designed to handle every kind,
form and volume of scrap material. The grinder must be the proper size to
handle the bulkiness of the scrap. IMS’ line of granulators, called the SHARXX
line, has a profile to fit any molder’s needs.
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Special attention should be paid to the cutting angle of the knives and the
screen used to ensure uniform particle size similar to that of virgin pellets.
The standard knife material is D-2. This is a good all-round mid-range tool
steel that does not lose its edge quickly.
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The screen must work properly. Loose or broken screens can result in oversized
pieces.
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Contamination
Let’s touch on contamination here. There are actually plants where they regrind
their burnt material – thinking that if they dilute it with enough virgin
material the mixture will work. Wake up! If you put junk in, you’ll get junk
out. All these plants produce is more junk material to send to the grinder.
Whatever the case, it is imperative that processors keep regrind clean and
treat it with the same care as virgin resin. The best way to ensure cleanliness
and avoid contamination is to place all regrind-ready plastic in clean gaylords
and move it to a separate room for grinding. |
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| Gaylord liners can safeguard the material from
contamination at minimal cost. You can use the liners once and throw them out.
IMS now stocks Gaylord liners for immediate shipment. |
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Degraded Polymer
What about adding this to the regrind? To maximize the use of regrind, first
and foremost, one needs to know how many times a material can go through the
molding process before its molecular, lubricating and stabilizing properties
are reduced to the point of being unusable. If you have questions, ask your
resin rep. |
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Mixing Regrind with Virgin Resins
It is generally accepted that using up to 25% regrind does not significantly
compromise the properties of most virgin resins. Regrind has the potential of
going through the molding process hundreds of times and, yes, the signs of
degradation will eventually show up in your product. Also, remember that every
time you blend regrind with virgin the properties of your regrind/virgin
mixture change. This chemical change may not be visible to the naked eye. In
fact, resulting problems may not even be discovered until the parts are in
service, which can become a costly issue. |
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| Further, each processing history may influence the
virgin resin’s additive levels and, if fiber filled, the length of the fibers.
Processing grinds up the fibers to shorter lengths. Additionally, problems can
occur when virgin is processed at higher than recommended temperatures — a sure
way to accelerate polymer degradation. All of these potential problems can
narrow the processing window. Finally, the ratio of virgin and uniform regrind
must be consistent. Some people say weighing is the best way of maximizing
consistency, others say volume (e.g. one bucket regrind per 4 buckets virgin).
There are arguments to both sides. However you decide to set mixture
proportions: BE CONSISTENT. |
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Noise
Granulators can be noisy, especially if the material being ground is hard. Even
with the sound-proofing being applied to today’s granulators, the noise can be
irritating. Having a room designated only for grinding will be of benefit to
reduce both noise and potential material contamination. |
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A Final Note
Many molders place their granulators next to the molding machines. There are at
least four reasons why that should be avoided: |
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Fines generated during regrinding can potentially contaminate the process and
add to unsightly areas around the molding area.
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Grinding machinery which is not centrally or separately located increases the
noise level on the production floor.
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If the parts being ground are not cool enough, the grinder may clog and the
knives may wear quickly.
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Inconsistent granular size feeding back into the process can actually cause
erratic molding cycles, adding to scrap.
|
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| With resin prices climbing, the use of regrind
continues to be an issue of importance for the molder. Whether a blessing or a
plague, regrind will always be an integral part of the molding industry. We
will always have scrap plastic to dispose of. Fortunately, the cost of
granulator purchases and their upkeep can be significantly offset by the wise
use of regrind in the molding process. Following the above general guidelines
will make your voyage into this area less troublesome. |
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| Sharxx Line™ Granulators related to this article: |
Goblin 614 and 814 Models |
|
Great White 2436 and 2442 Models |
|
Hammerhead 1824, 1836 and 1848 Models |
|
Mako 1012, 1018 and 1024 Models |
|
Megamouth 1418, 1424 and 1436 Models |
|
Pygmy 69, 614 and 617 Models |
|
Reef 89, 814, 819 and 824 Models |
|
Spinner 99, 913 and 918 Models |
|
Thresher 1224 Model |